We do not yet know if there might be such a planet. Since the universe seems to be so vast, most scientists guess that that there might be at least a few. The relation between that planet and its sun would have to be exactly right in order for there to be water and moderate temperatures, both of which are required for life. The size of the planet would need to be very like that of Earth in order to have just the right amount of gravity. If the planet was bigger, animals and trees would not be able to get very big because of the increased gravity. A bigger or smaller planet could have life of some kind, like bacteria or single celled plants. On a very big planet, living things would be very flat against the ground, and not move very much, like lichens. This would also be the case for plantlike life if the sunlight on that planet was much weaker than it is on ours.

Some evolutionary biologists have considered the question: ‘What if evolution on Earth were to be re-run? Would the results be the same kinds of beings as did evolve, or would they be very different?’ The current theory of evolution emphasizes that evolutionary changes result from accidental events like mutations. From that point of view we would expect many differences from our current floras and faunas in the re-run. But some things would necessarily be similar, like swimming animals would have to be streamlined. But even now we have exceptions, like swimming turtles, rays and crabs. And plants would have to be spread out to catch sunlight. For example, the number of legs for walking could not be predicted very well. On Earth now we have many differences in this, ranging from two to a hundred or so (in centipedes).

Since the origin of life is a complete mystery at present, some scientists think it might be such a rare event that it may have happened only once. Scientists who favor lawful prediction would tend to guess that the results of evolution on another planet would be very similar, while those who see the world as ruled by chance would guess the opposite.